This invention generally relates to de-energized tap changers used in power transformers. More particularly, the invention relates to a de-energized tap changer incorporating a self-aligning transmission system.
Power transformers will typically utilize a de-energized tap changer, sometimes called a no-load tap changer, to make adjustments to the primary windings ratio. Such adjustments are necessary to compensate for long-term line voltage variations that depend on the physical distance of the transformer from the power source. This adjustment is initially made upon installation and further adjustments are usually only necessary a few more times over the life of the transformer based upon changes in the power system to which the transformer is connected.
Basically, a de-energized tap changer works by either increasing or decreasing the number of transformer windings utilized. Typically, the device will affect the number of windings on the primary side of the transformer, that is, the side of the transformer receiving power from the power source. Most de-energized tap changers are designed to operate around a five step switch. It is common practice to place the nominal system voltage as the middle step and split the remaining four steps into two groups of taps that will step up or step down the voltage usually by two and one-half percent for each step. This is the typical set-up as the American National Standards Institute recommends the utility supply a voltage that will not vary by more than five percent from the nominal system voltage.
A de-energized tap changer will typically include a series of fixed contacts, connected to the taps of the primary winding of the transformer, which are within the transformer tank and immersed in the transformer oil. A movable contact is driven between the contacts through a drive mechanism connected to a manually operated handle outside the tank of the transformer. Switching of tap positions is effected when there is no power being supplied to the transformer.
As mentioned, these changes are usually effected by a drive mechanism connected to a manually operated handle outside the tank of the transformer. The various positionings for each step, either up or down, will be marked on the outside of the tank such that the handle, and the contacts attached to it, can be properly aligned. As should be apparent, when the de-energized tap changer is installed it should be properly aligned such that with each position change of the handle on the outside of the tank, the proper corresponding positioning of the contacts is achieved on the interior of the tank. Thus, installation of a de-energized tap changer becomes a difficult task to undertake in light of having to align the contacts, as well as having to create a hole on the exterior of the tank whereby the handle of the de-energized tap changer may protrude. Currently, some de-energized tap changers utilize a universal joint to compensate for misalignment of the handle and the rest of the assembly. Such joints typically have a cross-shaped piece having pivots on its arms so arranged that each pair of pivots engage with the eyes of a yoke on the end of one shaft. While a universal joint may remedy some of the problems set forth above, they have a major drawback in that they can be very expensive. Therefore, a less complicated and less-expensive device is needed such that the degree of precision presently required upon installation may be lessened.
The tap changer of the present invention is designed to remedy the need for exact precision upon installing a de-energized tap changer and thus simplify the arduous task of installing the device. One presently preferred embodiment of the present invention achieves this goal by providing a de-energized tap changer which includes a contact rail assembly, a gear assembly, a handle assembly, and a transmission system which compensates for angular misalignment of the gear assembly and handle assembly. The transmission systems includes two elongated members which extend from the handle assembly and the gear assembly and are connected via an elongated hollow tube.
Other aspects of the present invention are described below.